ABSTRACT Tobacco is the leading preventable cause of death in the United States, affecting both tobacco users and those exposed to second- and third hand smoke. Residents of low-income, multiunit housing are particularly vulnerable to tobacco smoke exposure. Low-income individuals are more likely to smoke, which in turn increases the tobacco smoke exposure of people sharing their environments. In November of 2015, the U.S. Department of Housing and Urban Development (HUD) proposed a rule requiring federally-funded public housing authorities (PHAs) to establish smoke-free policies for their residences, potentially reducing the morbidity associated with smoking and tobacco smoke exposure. Massachusetts PHAs have been at the forefront of smoke-free housing policies and their experience provides an early opportunity to assess the health benefits of HUD's proposed smoke-free policy, as well as smoke-free policies in other low-income housing. The principal investigator and colleagues have shown that public housing developments with smoke- free policies have lower levels of tobacco smoke pollution in the air (fine particle matter [PM2.5], nicotine) than those without smoke-free policies. In addition, they found that after the Boston Housing Authority introduced its smoke-free policy in 2012, airborne nicotine in residents' apartments went down by 70%. While the evidence that smoke-free housing policies reduce tobacco smoke pollution in PHAs is promising, the existing research has notable limitations. Furthermore, no studies have yet examined changes in health outcomes for residents following the introduction of smoke-free housing policies. Smokers and non-smokers alike are expected to benefit from the prohibition of smoking in multiunit housing. We propose assessing the potential health impact of smoke-free public housing policies on tobacco treatment use, signal adverse clinical events, and health care spending for all public housing residents in Massachusetts. Our study will use a strong quasi-experimental design to evaluate a series of natural experiments arising from the staggered introduction of smoke-free policies in ?27% of the state's 241 PHAs. Residents in PHAs that do not adopt smoke-free policies will serve as concurrent controls. We will measure outcomes for PHA residents using 2008-2018 data from the Massachusetts All Payer Claims Database (APCD), a uniquely comprehensive resource that includes health care claims for all insured residents from nearly all health insurers in the state. In addition, we will include data on tobacco treatment use from the Massachusetts state quit line. Findings from this study will have policy- relevant implications for smoke-free housing in state-funded PHAs (not covered by HUD's policy), other subsidized and low-income housing, and private housing markets nationally.